Terbuthylazine
(endorsed 2011)
Guideline
Based on human health concerns, terbuthylazine in drinking water should not exceed 0.01 mg/L.
Related chemicals
Terbuthylazine (CAS 5915-41-3) is in the triazine class of chemicals. Other pesticides in this class include ametryn, atrazine, simazine and terbutryn (Tomlin 2006).
Human risk statement
With good water quality management practices, the exposure of the general population is expected to be well below levels that may cause health concerns.
If present in drinking water as a result of a spillage or through misuse, terbuthylazine would not be a health concern unless the concentration exceeded 0.01 mg/L. Minor excursions above this level would need to occur over a significant period to be a health concern, as the health-based guideline is based on long-term effects.
With good water quality management practices, pesticides should not be detected in source waters used for drinking water supplies. Persistent detection of pesticides may indicate inappropriate use or accidental spillage, and investigation is required in line with established procedures in the risk management plan for the particular water source.
General description
Uses: Terbuthylazine is a herbicide used to control pre- and post-emergent weeds in a variety of agricultural crops and in forestry. It is also used as an industrial algicide in water cooling systems, ponds and fountains, and is used as an algicide in swimming pools.
There are currently no registered products containing terbuthylazine in Australia, but de-registered compounds may still be detected in water. Terbuthylazine has been used previously by professionals and applied by boom spray to crops as well as being used in swimming pools. Data on currently registered products are available from the Australian Pesticides and Veterinary Medicines Authority.
Exposure sources: The main source of public exposure to terbuthylazine, if used, would be residues in food.
If used in the future in agriculture, its use may potentially lead to contamination of source waters through processes such as run-off, spray drift or entry into groundwater.
Typical values in Australian drinking water
Values for terbuthylazine in Australian drinking water were not found. Levels in surface and bore waters in Europe and North America ranged from less than 0.1 µg/L to approximately 2 µg/L. Levels in Australian waters would normally be expected to be within this range.
Treatment of drinking water
Relatively high removal rates of terbuthylazine have been achieved using conventional flocculation, adsorption onto activated carbon and ozonoation (Ormad et al. 2008). If terbuthylazine is detected, jar testing with a matrix of multiple oxidants, adsorbents, and coagulants is recommended.
Measurement
Measurement of residue levels in water can be undertaken by use of solid phase extraction and high performance liquid chromatography with either ultraviolet detection or tandem mass spectrometry.
History of the health values
The current acceptable daily intake (ADI) for terbuthylazine is 0.003 mg per kg of bodyweight (mg/kg bw), based on a no-observed-effect level (NOEL) of 0.35mg/kg bw/day from a 24-month dietary study in rats showing a decrease in bodyweight gain and food consumption. The ADI incorporates a safety factor of 100, and was established in 2001.
A health value has not been previously established by NHMRC.
Health considerations
Metabolism: Terbuthylazine is readily absorbed from the gastrointestinal tract (~ 90%) and distributed widely in tissues. It is extensively metabolised and excreted rapidly, mainly via the urine, within 7 days. There is no evidence of bioaccumulation.
Acute effects: Terbuthylazine has low acute oral and dermal toxicity. There is some evidence for skin sensitisation in guinea pigs.
Short-term effects: In dietary studies in rats, there was reduced bodyweight gain at 75 mg/kg bw/day in rats. There was no effect on organ weights or tissue histopathology at doses up to 5 mg/kg bw/day in rats.
Long-term effects: In long-term dietary studies in mice, rats and dogs, there was a decrease in bodyweight gain and food consumption in all species. There were no consistent changes in haematology, clinical chemistry or histopathological findings indicative of systemic toxicity. The lowest NOEL was 0.35 mg/kg/day in the 2-year rat study, based on reduced bodyweight gain and reduced food consumption. This NOEL is the basis for the ADI.
Carcinogenicity: Based on long-term studies in mice and rats, there is no evidence of carcinogenicity for terbuthylazine.
Genotoxicity: Terbuthylazine is not considered to be genotoxic, based on in vitro and in vivo short-term studies.
Reproductive and developmental effects: In a reproduction study in rats and developmental studies in rats and rabbits, terbuthylazine showed no evidence of effects on reproductive parameters or on foetal development.
Poisons Schedule: Terbuthylazine is included in Schedule 6 of the of the Standard for the Uniform Scheduling of Medicines and Poisons No.1, 2010 (the Poisons Standard)(DoHA 2010), depending on the concentration and use. Current versions of the Poisons Standard should be consulted for further information.
Derivation of the health-based guideline
The health-based guideline of 0.01 mg/L for terbuthylazine was determined as follows:
where:
0.35 mg/kg bw/day is the NOEL based on a long-term (2-year) dietary study in rats.
70 kg is taken as the average weight of an adult.
0.1 is a proportionality factor based on the assumption that 10% of the ADI will arise from the consumption of drinking water.
2 L/day is the estimated maximum amount of water consumed by an adult.
100 is the safety factor applied to the NOEL derived from animal studies. This safety factor incorporates a factor of 10 for interspecies extrapolation and 10 for intraspecies variation.
The World Health Organization has a guideline value of 0.007 mg/L for terbuthylazine (WHO 2004).
References
NOTE: The toxicological information used in developing this fact sheet is from reports and data held by the Department of Health, Office of Chemical Safety.
DoHA (2010) The Poisons Standard; Schedule 1-Standard for the Uniform Scheduling of Medicines and Poisons, Department of Health and Ageing, Commonwealth of Australia, Canberra.
Ormad MP, Miguel N, Claver A, Matesanz JM, Ovelleiro JL (2008). Pesticides removal in the process of drinking water production. Chemosphere, 71: 97–106.
Tomlin CD (ed) (2006). The Pesticide Manual: a world compendium, 14th Edition, British Crop Production Council, UK.
WHO (World Health Organization) (2004). Guidelines for Drinking-water Quality. 3rd Edition, WHO, Geneva, Switzerland.
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